The transferrins are a class of two-sited iron-binding proteins which function to transport iron in physiologic fluids and across physiologic boundaries. Serum transferrin, the prototype and most widely studied of this class, is the specific source of iron for the biosynthesis of hemoglobin by immature red blood cells. The proposed research is concerned with the physical and chemical events in the binding of iron by transferrin, and the release of iron to the reticulocyte from the protein. Since iron is tightly bound by transferrin only in the presence of bicarbonate or other suitable anions, C13-nuclear magnetic resonance studies of the anion-binding function of the protein are planned. The particular questions to be approached concern the functional and structural identity of the two anion-binding sites and the proximity of each of its associated iron- binding site. Studies are also planned of the transferrin-binding receptors of the reticulocyte and the mechanism of their interaction 1ith transferrin.